Microfilm camera operators engaged, for example, in preservation recordation of materials, traditionally have been required to subjectively evaluate the exposure requirements for the multiple pages of such works and adjust their equipment accordingly. Although the filming environment is more stable for microfilming than is common in general photography, efforts in the former endeavor must be undertaken under rigid standards, for example those established by the Research Library Group (RLG). These standards require that the background in the final product image exhibit a uniform "density" within a very tight range tolerance. To achieve that requisite consistency, variations in the light and color characteristics of the subject material e.g. "fading" or yellowing or browning of paper by oxidation must be compensated for. Even small, subjectively imperceptible variations become significant, the darker pages near the beginning or end of a decaying book requiring different exposure than needed for the lighter pages near the middle. Where exposure evaluation fails, the work must be repeated and acceptable replacement frames spliced into a film reel. The noted standards limit the permissible number of such splicings per reel. Thus, the filming of an entire reel of 600 to 1200 frames may be required to be repeated should the operator's evaluation of correct exposure fail excessively.
A typical microfilming production station is somewhat basic. The camera head is movably supported upon a column extending above a work surface. Conventionally, four photographic flood lamps are mounted adjacent to and above the work surface. The luminal output of these lamps is controlled by a rheostat mounted with the assembly such that the light value therefrom is regulated in terms of volts. While the exposure setting can be established by conventional shutter speed-aperture adjustment, the photographic parameter adjustment for the system conventionally is provided by simply setting the "volts" indicator on the rheostat or at an associated voltmeter in conjunction with the provision of fixed values for shutter speed and aperture. The assembly also will include a conventional exposure meter typically mounted adjacent the supporting column and arranged to view and respond to an averaged or integrated value for reflective brightness at the subject plane or filming position. That plane typically will provide for positioning the work to be photographed such as a page or paired adjacent pages of an open book beneath a glass cover. Generally, an operator will film as many as 250 frames in an hour during a typical work day.
The exposure meters employed with microfilming stations generally are inadequate for the purpose of achieving proper exposure adjustment. Variations in the print content and style on a given page to be filmed will cause error in reflectance evaluations otherwise made by the meters. Thus, a conventional approach to determining correct exposure settings has been to carry out a process of test filming of the subject material under different exposure settings. The resultant film then is developed and, the test images are evaluated, e.g. using a transmittance densitometer to determine which light or exposure parameter setting is appropriate. Once that setting is determined, it becomes the task of the operator to evaluate "fade" or background reflectance changes, for example, as the pages of a book are turned from the beginning of the book towards the middle and from the middle of the book towards its end. Often eye fatigue and the like enters to lend confusion to this personal and subjective evaluation. Such test filming approaches are slow and costly, error prone and inflexible. Further, the testing must be repeated when the subject material changes, when the camera set-up changes, as by lamp replacement, and when work is moved from one camera or camera operator to another. Cameras and their support equipment exhibit subtle variations from one station to another (e.g. shutter, lens and lamp characteristics) and ambient light conditions often vary among work stations. Thus, the exposure settings for one work station have no validity for another.
One normalizing approach to the test filming procedure has been to employ a pseudo background which may be provided, for example, as a card having a hue such as a beige color serving to somewhat emulate a conventionally aged page of material. With this approach, exposure meter settings were established using a beige card at the target position for the camera. The card was then removed and a test filming of the subject work was then made at a light voltage setting adjusted in correspondence with the earlier determined exposure meter setting. This procedure was carried out over a sequence of exposure increments and the resultant film images then were evaluated using a noted transmittance densitometer approach.